Arched saddle-shaped NMR RF coils

ABSTRACT

According to one aspect, a nuclear magnetic resonance (NMR) radio-frequency (RF) coil of an NMR spectrometer includes oppositely-facing arches at opposite longitudinal ends of the coil window, along shielded regions of the coil. The arched coil shapes allow situating the transverse parts of the coil away from the interfaces between the sample measurement volume and coil shields while minimizing the additional coil length and thus coil inductance and capacitance needed to move the transverse conductors away from interfaces. Moving the transverse parts of the coil away from the measurement volume facilitates shimming. The coil may be lifted on a plurality of longitudinal support rods spaced around a circumference of the coil, in order to reduce the capacitance between the coil and associated shields.

FIELD OF THE INVENTION

The invention in general relates to nuclear magnetic resonance (NMR)spectroscopy, and in particular to NMR radio-frequency (RF) coils.

BACKGROUND OF THE INVENTION

Nuclear magnetic resonance (NMR) spectrometers typically include asuperconducting magnet for generating a static magnetic field B₀, and anNMR probe including one or more special-purpose radio-frequency (RF)coils for generating a time-varying magnetic field B₁ perpendicular tothe field B₀, and for detecting the response of a sample to the appliedmagnetic fields. Each RF coil and associated circuitry can resonate atthe Larmor frequency of a nucleus of interest present in the sample.Nuclei of interest analyzed in common NMR applications include ¹H(proton), ¹³C (carbon), and ¹⁵N (nitrogen). The RF coils are typicallyprovided as part of an NMR probe, and are used to analyze samplessituated in sample tubes or flow cells. The direction of the staticmagnetic field B₀ is commonly denoted as the z-axis or longitudinaldirection, while the plane perpendicular to the z-axis is commonlytermed the x-y or transverse direction.

Several types of RF coils have been used in NMR systems. In particular,many NMR systems include transverse-field RF coils, which generate an RFmagnetic field oriented along the x-y plane. Transverse-field coilsinclude saddle-shaped coils and birdcage coils. Birdcage coils typicallyinclude two transverse rings, and a relatively large number of verticalrungs connecting the rings. Birdcage coils are multiply-resonantstructures in which specified phase-relationships are established forcurrent flowing along multiple vertical rungs. Saddle-shaped coilsnormally have the current path defined by a conductor pattern around thecoil windows.

An NMR frequency of interest is determined by the nucleus of interestand the strength of the applied static magnetic field B₀. In order tomaximize the accuracy of NMR measurements, the resonant frequency of theexcitation/detection circuitry is set to be equal to the frequency ofinterest. The resonant frequency of the excitation/detection circuitryvaries asv=1/2π√{square root over (LC)}  [1]where L and C are the effective inductance and capacitance,respectively, of the excitation/detection circuitry.

Generating high-resolution NMR spectra is facilitated by employing atemporally and spatially-homogeneous static magnetic field. The strengthof the static magnetic field can vary over time due to temperaturefluctuations or movement of neighboring metallic objects, among others.Spatial variations in the static magnetic field can be created byvariations in sample tube or sample properties, the presence ofneighboring materials, or by the magnet's design. Minor spatialinhomogeneities in the static magnetic field are ordinarily correctedusing a set of shim coils, which generate a small magnetic field whichopposes and cancels inhomogeneities in the applied static magneticfield. Shimming is generally facilitated if the various components of anNMR probe introduce minimal inhomogeneities into the static magneticfield.

SUMMARY OF THE INVENTION

According to one aspect, a nuclear magnetic resonance radio frequencycoil assembly comprises a coil support, and a saddle-shaped nuclearmagnetic resonance radio-frequency coil mounted on the coil support, theradio-frequency coil defining a pair of opposite, laterally-facing coilwindows, each coil window including a pair of oppositely-facing archesat opposite longitudinal ends of said each coil window.

According to another aspect, a nuclear magnetic resonance apparatuscomprises a magnet for applying a static magnetic field to a sample; ashim coil coupled to the magnet, for reducing a spatial inhomogeneity ofthe static magnetic field; and a saddle-shaped nuclear magneticresonance radio-frequency coil coupled to the magnet, for applying aradio-frequency magnetic field to the sample, the radio-frequency coildefining a pair of opposite, laterally-facing coil windows, each coilwindow including a pair of oppositely-facing arches at oppositelongitudinal ends of said each coil window.

According to another aspect, a nuclear magnetic resonance measurementmethod comprises employing a saddle-shaped nuclear magnetic resonanceradio-frequency coil to apply a set of radio-frequency pulses to asample, the radio-frequency coil defining a pair of opposite,laterally-facing coil windows, each coil window including a pair ofoppositely-facing arches at opposite longitudinal ends of said each coilwindow; and detecting a response of the sample to the set ofradio-frequency pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and advantages of the present invention willbecome better understood upon reading the following detailed descriptionand upon reference to the drawings where:

FIG. 1 is a schematic diagram of an exemplary NMR spectrometer accordingto some embodiments of the present invention.

FIG. 2-A shows an isometric view of a radio-frequency (RF) NMR coilassembly including an arched, saddle-shaped RF coil according to someembodiments of the present invention.

FIG. 2-B shows an isometric view of a support/shielding structure of theRF coil assembly of FIG. 2-A, according to some embodiments of thepresent invention.

FIG. 3 shows a patterned foil used to make a coil such as the one shownin FIG. 2-B upon rolling about a longitudinal axis, according to someembodiments of the present invention.

FIG. 4-A shows an exemplary RF coil arch shape with circular curvature,according to some embodiments of the present invention.

FIG. 4-B shows an exemplary RF coil arch shape having an ellipticalcurvature, according to some embodiments of the present invention.

FIG. 4-C shows an exemplary RF coil arch shape having an acute arch top,according to some embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, a set of elements includes one or moreelements. Any reference to an element is understood to encompass one ormore elements. Each recited element or structure can be formed by or bepart of a monolithic structure, or be formed from multiple distinctstructures. A longitudinally-monolithic foil is a foil that is notformed by connecting multiple longitudinally-separated parts; alongitudinally-monolithic foil may include multiple layers stacked alonga non-longitudinal direction. The statement that a coil is used toperform a nuclear magnetic measurement on a sample is understood to meanthat the coil is used as transmitter, receiver, or both. Unlessotherwise stated, any recited electrical or mechanical connections canbe direct connections or indirect connections through intermediarycircuit elements or structures. A conductive ring is a structure thatprovides a ring-shaped current path to RF current; such a structure caninclude two or three concentric, capacitively-coupled physical rings,some or all of which may include longitudinal slots; such physical ringscan be formed, for example, by part of a central foil, a capacitiveshield, and a capacitance band. A conductive ring can also include asingle, monolithic physical ring providing a ring-shaped path to DCcurrent. The statement that a longitudinal conductor electricallycouples two conductive rings is understood to mean that the longitudinalconductor provides a current path for RF current flowing between the tworings. Such a longitudinal conductor can be physically (resistively)connected to one or both of the rings (providing both DC and RF currentpaths), or capacitively coupled to one or both of the rings.

The following description illustrates embodiments of the invention byway of example and not necessarily by way of limitation.

FIG. 1 is a schematic diagram illustrating an exemplary nuclear magneticresonance (NMR) spectrometer 12 according to some embodiments of thepresent invention. Spectrometer 12 comprises a magnet 16, an NMR probe20 inserted in a cylindrical bore of magnet 16, and acontrol/acquisition system (console) 18 electrically connected to magnet16 and probe 20. Probe 20 includes nested radio-frequency (RF) coils 24a-b and associated electrical circuit components. In some embodiments,one of the coils 24 a-b is a proton coil, while the other is an X-coil.In some embodiments, probe 20 may include a single RF coil. A samplecontainer 22 is positioned within probe 20, for holding an NMR sample ofinterest within coils 24 a-b while measurements are performed on thesample. Sample container 22 can be a sample tube or a flow cell. A setof shim coils 30 laterally enclose coils 24 a-b.

To perform a measurement, a sample is inserted into a measurement spacedefined within coils 24 a-b. Magnet 16 applies a static magnetic fieldB₀ to the sample held within sample container 22. Shim coils 30 are usedto correct spatial inhomogeneities in the static magnetic field B₀.Control/acquisition system 18 comprises electronic components configuredto apply desired radio-frequency pulses to probe 20, and to acquire dataindicative of the nuclear magnetic resonance properties of the sampleswithin probe 20. Coils 24 a-b are used to apply radio-frequency magneticfields B₁ to the sample, and/or to measure the response of the sample tothe applied magnetic fields. The RF magnetic fields are perpendicular tothe static magnetic field. The same coil may be used for both applyingan RF magnetic field and for measuring the sample response to theapplied magnetic field. Alternatively, one coil may be used for applyingan RF magnetic field, and another coil for measuring the response of thesample to the applied magnetic field.

FIG. 2-A shows an isometric view of a radio-frequency (RF) NMR coilassembly 40 including a support and shielding assembly 44 and an archedsaddle-shaped RF coil 24 mounted on support/shielding assembly 44,according to some embodiments of the present invention. In the exemplaryillustration of FIG. 2-A, RF coil 24 is a two-turn coil.

On each of two opposite lateral sides (front and back in FIG. 2-A), RFcoil 24 includes two concentric loops 52 a-b enclosing alaterally-facing coil window 46. Current flows through loops 52 a-b inthe same direction (clockwise or counterclockwise), such that the RFmagnetic fields generated by the two loops reinforce each other. Thedirection of the RF magnetic field generated by coil 24 is generallyin/out of the plane of the paper in FIG. 2-A. Each loop 52 a-b includesstraight longitudinal conductors 80 a-b, and longitudinally-spaced,oppositely-facing connecting arches 82 a-b providing a transverseconnection between conductors 80 a-b. Longitudinal conductors 80 a-bextend along a sample measurement volume, while arches 82 a-b aresituated along a shielded longitudinal region of assembly 40. Each arch82 a-b follows and curves azimuthally around the lateral cylindricalsurface of support assembly 44.

FIG. 2-B shows an isometric view of support/shielding assembly 44.Assembly 44 includes a generally longitudinal, cylindrical insulativecoil support 50, and a pair of conductive shields 60 a-b disposed alongan internal lateral surface of coil support 50. Shields 60-a-b areformed by longitudinally-slotted cylindrical conductors disposed onopposite longitudinal sides of a measurement volume 64 accommodating asample of interest. The central axis of each shield 60 a-b is alignedwith the longitudinal central axis of assembly 44. Sample containers(sample tubes and/or flow cells) containing NMR samples of interest areinserted and removed sequentially through a longitudinal bore 48 definedby support assembly 44, to place the samples within measurement volume64. Shields 60 a-b serve to reduce the parasitic excitation of the NMRsamples due to RF pickup from coil leads or other conductive structures,and to shield the NMR samples from undesired external electric fields.Shields 60 a-b also shield the static magnetic field within measurementvolume 64 from the effects of current flow through arches 82 a-b.Shields 60 a-b may also provide additional distributed capacitance tocoil assembly 40.

Coil support 50 includes two longitudinally-spaced rigid support rings58 a-b, a cylindrical tube 54 extending between support rings 58 a-b,and a pair of longitudinal cylindrical conductor-spacer rods 62 a-badjacent to the external surface of tube 54 and extending betweensupport rings 58 a-b on opposite lateral sides of tube 54. Coil 24 isattached to rods 62 a-b. Rods 62 a-b lift coil 24 away from the surfaceof tube 54.

The materials and dimensions of the various components of assembly 44may be chosen to be compatible with desired NMR applications and/orsystems. In some embodiments, RF coil 24 a singlesusceptibility-compensated thin conductive foil. The foil can includeone or multiple layers of material, and is monolithic along its mainsurface plane. In some embodiments, RF coil 24 may be formed from asusceptibility-compensated Pd—Cu or Al—Cu—Al foil, and shields 60 a-bmay be made of susceptibility-compensated Pd-plated copper. Othermaterials having susceptibilities of opposite signs can be used to yielda magnetic susceptibility equal to the magnetic susceptibility of air orvacuum. For example, rhodium, platinum, copper and stacks of suchmaterials may be used for RF coil 24 and/or shields 60 a-b. Supportrings 58 a-b may be made of a rigid polymer such as PCTFE(polychlorotrifluoroethylene, known under the trade name Kel-F®). Insome embodiments, tube 54 is made of sapphire or quartz, and rods 62 a-bare made of glass. RF coil 24 may be formed from a conductive sheethaving thickness on the order of mils (milliinches, or about 2.5×10⁻⁵m), for example about 1 mil. The width of the conductors of RF coil 24may be on the order of mm, for example about 1 mm. Tube 54 may have adiameter on the order of cm, for example about 1 cm, and a longitudinalextent on the order of cm to tens of cm, for example about 10 cm. Rods62 a-b may have a diameter on the order of mm, for example about 1 mm.The longitudinal extent of coil 24 may be on the order of cm to tens ofcm, for example about 10 cm, while the longitudinal extent of each arch82 a-b may be on the order of cm, for example about 1-2 cm.

FIG. 3 shows a patterned foil used to make coil 24 (FIG. 2-A) uponrolling about a longitudinal axis, according to some embodiments of thepresent invention. The patterned foil includes two two-turn loops 86,86′, each corresponding to one lateral side (front/back in FIG. 2-A) ofcoil 24. Loop 86 includes linear (straight) longitudinal conductors 80a-b and oppositely-facing interconnecting arches 82 a-b connectingconductor 80 a to conductor 80 b. A longitudinal external connectionlead 88 bends back along conductor 80 b, and serves as one of the twoexternal terminals of coil 24. A set of temporary stabilization bridges90 a-b interconnect the outer and inner conductors of loop 86 a at thetips of arches 82 a-b, respectively. Bridges 90 a-b are defined duringthe patterning of coil 24, and are removed after the mounting of coil 24on support assembly 44 (FIG. 2-B).

In some embodiments, coil 24 is made by patterning a planarsusceptibility-compensated Al—Cu—Al trilayer foil. The patterned planarfoil is rolled about a longitudinal axis to form coil 24 and mounted onsupport assembly (FIG. 2B). Temporary stabilization bridges 90 a-b (FIG.3) are subsequently removed.

FIG. 4-A-C show three exemplary coil arch shapes according to someembodiments of the present invention. Arched coils 124, 224, 324 includelinear longitudinal conductors 180, 280, 380 and corresponding arches182, 282, 382, respectively. Longitudinal conductors 180 extend alongcorresponding measurement volumes, while arches 182, 282, 382 extendalong corresponding shielded regions. The interfaces between themeasurement volumes and shielded areas are schematically illustrated inFIGS. 4-A-C at 160, 260 and 360, respectively. As shown, coils 124, 224,324 are straight along interfaces 160, 260, 360, respectively. Arch 124has a circular curvature, arch 224 has an elliptical curvature, and arch324 is a gothic arch having an acutely-angled apex. Generally, a flatterdesign such as the one shown in FIG. 4-B may bring the transverse partof the coil relatively close to the shield/measurement volume interface,or, if the transverse part is moved further away from the interface, maylead to relatively long coil conductors along the shielded areas. A moreacutely-shaped design such as the one shown in FIG. 4-C allows movingthe transverse conductors away from the interface while introducing arelatively low additional coil length along the shielded area.

The exemplary arched RF coil designs described above allow achievingimproved shimming performance in the context of the complex and oftencompeting set of constraints that NMR RF coil designers generally takeinto account. Ideally, an NMR RF coil has a high quality factor (Q), aself-resonance frequency (SRF) well above the resonance frequency of thenucleus of interest, and is easy to shim so that the lineshape of thesample of interest is as narrow as possible. Using thick conductors(e.g. wires) for the coil may allow achieving high Q values, but mayincrease the difficulty of shimming. Shimming may be facilitated byforming the coil from a thin susceptibility-compensated foil, andadditionally by using shields that screen parts of the coil that areparticularly difficult to shim from the sample. Such shields generallyintroduce additional capacitance into the sample, which lowers the coilSRF and adversely affects the coil Q. The coil SRF may also be loweredby the introduction of additional coil conductor length, which increasesthe coil inductance and capacitance.

It has been observed that transverse conductors generally affectshimming and magnetic field homogeneity to a significantly larger extentthan smooth, linear longitudinal conductors. In particular, eventransverse conductors separated from a sample by a cylindrical shieldmay affect shimming. Effectively, the perturbation to the magnetic fieldcaused by transverse conductors can be felt at a distance. While thesample region screened by shields causes little or no line broadening,the sample region just inside the coil window (near the shield edge) maybe subject to a significant magnetic field perturbation if a transverseconductor is sufficiently close, resulting in a broader lineshape thatmay be difficult to shim out. An arched coil shape as described aboveallows situating the transversely-oriented parts of the coil away fromthe shield/measurement volume interface, where the coil conductors aresubstantially straight and longitudinal, without inordinately increasingthe coil length and thus the additional inductance and capacitanceintroduced by the shielded part of the coil. The coil capacitance isfurther reduced by lifting the coil on longitudinal rods as describedabove, thus separating the coil from the dielectric support. Generally,sapphire has a dielectric constant much larger (e.g. about an order ofmagnitude larger) than air/vacuum, and thus even a small gap between thecoil and the sapphire coil support tube allows substantially reducingthe capacitance between the shields and coil.

Shimming considerations may be particularly complicated in systems usingnested coils, such as dual-coil, dual broadband, and switchable probesystems. In such systems, the lineshape of the outer coil signals may besignificantly degraded by the presence of the inner coil. In systemsusing nested coils, the exemplary arched-coil designs described abovemay be particularly useful for use as inner coils. Nevertheless, sucharched coil designs may also be used in outer coils in nested coilsystems.

The above embodiments may be altered in many ways without departing fromthe scope of the invention. For example, an RF coil may include a wirerather than a foil; such a wire may be susceptibility compensated. Insome embodiments, larger numbers of lifting/support rods may be used,particularly if enhanced vibration dampening/control is desired.Accordingly, the scope of the invention should be determined by thefollowing claims and their legal equivalents.

1. A nuclear magnetic resonance radio-frequency coil assemblycomprising: a coil support; and a saddle-shaped nuclear magneticresonance radio-frequency coil mounted on the coil support, theradio-frequency coil defining a pair of opposite, laterally-facing coilwindows, each coil window including a pair of oppositely-facing archesat opposite longitudinal ends of said each coil window.
 2. The coilassembly of claim 1, further comprising a pair of shields situated onopposite longitudinal sides of a measurement volume, each arch of thepair of arches being situated along a corresponding shield of the pairof shields.
 3. The coil assembly of claim 2, wherein the radio-frequencycoil defines substantially straight and longitudinal current paths alonga pair of interfaces between the corresponding pair of shields and themeasurement volume.
 4. The coil assembly of claim 1, wherein the coilsupport comprises a plurality of coil support rods spaced apart along acircumference of a cylinder, the radio-frequency coil being mounted onthe plurality of coil support rods.
 5. The coil assembly of claim 1,wherein each of the arches has an acutely-angled apex.
 6. The coilassembly of claim 1, wherein the radio-frequency coil is formed from asingle, longitudinally-monolithic patterned foil.
 7. The coil assemblyof claim 1, wherein the radio-frequency coil comprises at least two coilturns.
 8. A nuclear magnetic resonance apparatus comprising: a magnetfor applying a static magnetic field to a sample held in a samplevessel; a shim coil coupled to the magnet, for reducing a spatialinhomogeneity of the static magnetic field; and a saddle-shaped nuclearmagnetic resonance radio-frequency coil coupled to the magnet, forapplying a radio-frequency magnetic field to the sample, theradio-frequency coil defining a pair of opposite, laterally-facing coilwindows, each coil window including a pair of oppositely-facing archesat opposite longitudinal ends of said each coil window.
 9. The apparatusof claim 8, further comprising a pair of shields situated on oppositelongitudinal sides of a measurement volume, each arch of the pair ofarches being situated along a corresponding shield of the pair ofshields.
 10. The apparatus of claim 9, wherein the radio-frequency coildefines substantially straight and longitudinal current paths along apair of interfaces between the corresponding pair of shields and themeasurement volume.
 11. The apparatus of claim 8, further comprising aradio-frequency coil support comprising a plurality of coil support rodsspaced apart along a circumference of a cylinder, the radio-frequencycoil being mounted on the plurality of coil support rods.
 12. Theapparatus of claim 8, wherein each of the arches has an acutely-angledapex.
 13. The apparatus of claim 8, wherein the radio-frequency coil isformed from a single, longitudinally-monolithic patterned foil.
 14. Theapparatus of claim 8, wherein the radio-frequency coil comprises atleast two coil turns.
 15. A nuclear magnetic resonance methodcomprising: employing a saddle-shaped nuclear magnetic resonanceradio-frequency coil to apply a set of radio-frequency pulses to asample, the radio-frequency coil defining a pair of opposite,laterally-facing coil windows, each coil window including a pair ofoppositely-facing arches at opposite longitudinal ends of said each coilwindow; and detecting a response of the sample to the set ofradio-frequency pulses.
 16. The method of claim 15, further comprisingshielding the sample from the arches using a pair of shields situated onopposite longitudinal sides of a measurement volume containing thesample.
 17. The method of claim 16, wherein the radio-frequency coildefines substantially straight and longitudinal current paths along apair of interfaces between the corresponding pair of shields and themeasurement volume.
 18. The method of claim 15, wherein theradio-frequency coil is mounted on a plurality of coil support rodsspaced apart along a circumference of a cylinder.
 19. The method ofclaim 15, wherein each of the arches has an acutely-angled apex.
 20. Themethod of claim 15, wherein the radio-frequency coil is formed from asingle, longitudinally-monolithic patterned foil.
 21. The method ofclaim 15, wherein the radio-frequency coil comprises at least two coilturns.
 22. The method of claim 15, further comprising applying to thesample a static magnetic field and a shimming magnetic field to reduce aspatial inhomogeneity of the static magnetic field.
 23. A planarpatterned foil defining a saddle-shaped nuclear magnetic resonanceradio-frequency coil when rolled about a longitudinal axis, theradio-frequency coil defining a pair of opposite, laterally-facing coilwindows, each coil window including a pair of oppositely-facing archesat opposite longitudinal ends of said each coil window.